Expose Preview Flag to Clientside
Use Preview Flag to Block Video Ads

Business requirement is to avoid making calls
to the ad server in the preview environment.
Merge branch 'master' into kc-video-preview

Conflicts:
	common/app/assets/javascripts/modules/commercial/video.js
Merge branch 'master' into kc-video-preview
Remove Redundant Script
Merge branch 'master' into kc-video-preview
Merge pull request #5802 from guardian/kc-video-preview

Block Video Ads in Preview Environment